(1) Field of the Invention
This invention relates to the saccharification of starchy substances, particulary to the method for glucose production from raw-starch using the enzyme produced by a basidiomycete belonging to the genus Corticium.
(2) Description of the Prior Art
On the manufacturing process of glucose, starchy substances have been saccharified by two-step reactions using two different types of amylases. The process consists of cooking process of starchy substances at elevated temperature, liquefaction of cooked starch by .alpha.-amylase at a temperature between 80.degree. and 120.degree. C., and saccharification by glucoamylase at a temperature between 45 .degree. and 60.degree. C.
This method requires a large amount of thermal energy in the preceding cooking process and some complicated procedures because the optimum temperatures of .alpha.-amylase and glucoamylase, and the optimum pHs of these enzymes are different, respectively. Furthermore, the cooked starch is so pasty that the enzyme reaction do not proceed uniformly at the concentration between 30 and 50%. From the standpoint of the control of factory, the process involves many difficult problems to be solved.
In order to overcome the above mentioned drawback, it has been made many attempts to saccharify raw-starch without cooking. For this purpose, the enzymes produced by the fungi belonging to the genus Aspergillus and the genus Rhizopus were frequently used. In these methods, glucose can be obtained by one-step reaction and the reaction mixture shows low viscosity even at a higher concentration of starch than 30%. Therefore, the methods provide some advantages in energy cost and operation techniques.
However, most enzymes used in these processes have less activity toward raw starch at a high substrate concentration. According to the report by Ueda et al on the enzymes of Aspergillus awamori [Starch, 33(9), 313(1981)] and Rhizopus sp. [Starch, 27, 123(1975)], an upper limit of starch concentration for the saccharification is 2% at the most. Moreover, the enzyme produced by Chalara paradoxa, which was reported by Kainuma et al as an active enzyme capable of saccharifying raw starch ["Dempun Kagaku" (Starch Science), 32(3), 189(1985) and Japanese Laid-open patent application No. 59-140896] was able to hydrolyze raw starch only at a concentration less than 5%. These results suggest that enzymatic saccharification of raw starch at a concentration more than 10% is very difficult.
In order to saccharify uncooked starchy substances, attention should be paid to prevent the contamination of various microbials during the reaction. The reaction should be carried out at a temperature as high as possible, preferably at a temperature between 45.degree. and 60.degree. C. The enzymes produced by Rhizopus sp. and Chalara paradoxa are inadequate for the saccharification of raw-starch, because they cannot be used at a temperature higher than 50.degree. C.
In recent years, it has been reported that an enzyme produced by a strain of Corticium rolfsii (IFO 4878) has some saccharifying activity toward uncooked starch [Nippon Shokuhin Kogyo Gakkaishi, 25 (1), 22 (1978)]. This enzyme has the activity to saccharify about 90% of starch in the reaction mixture containing 10% corn starch (w/v). As they stated in the report, it is believed that there is a limit to achieve effective saccharification of uncooked starch by the enzyme.
As it has been verified by the above mentioned references, there has never been a suitable method for industrial saccharification of starch without cooking.